R - This number represents
how many billions of stars in the galaxy meet the following two criteria:

The star must be a second
or third generation star formed from an interstellar cloud that included
the necessary heavy elements for life (e.g., carbon, oxygen, etc.).
The elements are created during the evolution of first generation, super-massive
stars and supernova events that occurred early in the history of our
galaxy. A reasonable estimate for this number is 400 billion stars.

The star must release enough
energy to have a sizeable habitable zone. A habitable zone is the region
around a star where liquid water could exist on an orbiting planet.
90% of the stars in our galaxy are too cool to have a sizable habitable
zone. This eliminates stars with spectral type K5 and cooler. Of the
remaining 10%, nearly a quarter of those have lifetimes too short for
life to develop. This eliminates stars warmer with spectral type F8
and warmer as they have lifetimes shorter than 4 billion years.

Our Sun, a G2 star, fits both
of these categories and thus is one of the target stars. Such target stars
are often referred to as Sun-like stars. A reasonable estimate for the
number of target stars is
400e9 * 10% * 75% = 30 billion stars.

fp
- This number represents
the fraction of those stars meeting the above criteria that also have
planets or planet systems around them. Recent discoveries of numerous
extra-solar planets suggest that most stars like our Sun probably have
planets.

ne- This number represents how many "earth-like planets"
there are at the right temperature for liquid water to exist (i.e. in
the habitable zone). Recent discoveries suggest that we should also
consider including moons around gas giant planets that are orbiting
their central star in the habitable zone. A reasonable estimate for
this number is difficult to imagine. In our solar system, the number
ranges from one to three depending on if you include Venus or Mars.
If Saturn were to migrate into the habitable zone, its 22 moons would
make this number much larger.

fl
- This number represents
the fraction of earth-like planets where life actually develops. Some
scientists believe that the evolution of life is inevitable when the
conditions are right. Alternatively, we only know of one instance where
life has successfully developed (Earth), therefore it is difficult to
estimate this fraction.

fi
- This number represents
the fraction of earth-like planets where at least one species of intelligent
life evolves. Intelligent life could develop early on some planets and
later on others and therefore again it is difficult to estimate this
fraction.

fc
- This number represents the fraction of earth-like planets where the
technology to communicate beyond the planet exists. In our own civilization,
we have been using television and radio signals for nearly a century.
These signals have leaked into outer space and might be detectable by
extraterrestrial civilizations. As before, it is extremely difficult
to estimate this number.

L - This number represents
the number of years that communicating civilizations have existed out
of the total lifetime that the galaxy has existed. We call this fraction
of years "Lifetime." This number depends both on social issues
and technological issues. It is possible that intelligent civilizations
elsewhere in the galaxy have existed for millions of years and may or
may not choose to communicate beyond their own planet. Alternatively,
when civilizations develop the technology to communicate they might
simultaneously develop technology capable of making their environment
uninhabitable (e.g., weapons of mass destruction). These factors make
this number extremely difficult to estimate. L could range from only
100 years to many millions of years.